The present invention relates to peak metering circuits, and more particularly to a dual function peak metering circuit for performing both monitoring and measuring functions in the presence of an input signal having both a modulated subcarrier component and an unmodulated baseband component. A peak metering circuit is used to determine peak signal levels for an input signal, and is used in various types of equipment, such as the 751 BTSC Aural Modulation Monitor/Decoder manufactured by Tektronix, Inc. of Beaverton, Oreg. Knowledge of peak levels is necessary for the two basic functions, monitoring and measuring, that put differing requirements on the peak metering circuit. Monitoring requires that the peak indication be weighted or lowpass filtered to ignore peaks that are of no informational consequence. Measuring requires that peak levels be quickly and accurately indicated in order to evaluate the response of the input signal source to test signals. An additional requirement of both functions is that response to modulated subcarrier components of the input signal be the same as response to unmodulated baseband components.
An example of a signal having multiple signal components is a BTSC (Broadcast Television Systems Committee) multi-channel television sound (MTS) signal that contains unmodulated, amplitude modulated and frequency modulated components in a bandwidth of zero to 120 kHz. An amplitude versus frequency graph of such a signal is shown in FIG. 1 where an L+R component is unmodulated, a pilot signal is unmodulated and an L-R component is suppressed carrier amplitude modulated. The resulting composite signal is shown in FIG. 2, with the L-R component shown separately in FIG. 3. As a result of BTSC specified companding time constants and filter ringing, the composite signal shows short duration transient artifacts. As shown in FIG. 4 when an audio burst test signal is input to such a system, the output composite signal has an artifact at the lead end that exceeds the steady state peak value. It is desired to ignore the peak value of the artifact but still correctly measure the peak of the steady state signal. Peak detectors with long time constants can successfully ignore the artifacts, but require a long time to settle to the steady sate value. Also, the indication for a modulated signal, as shown in FIG. 5, is less than that for an unmodulated signal, as shown in FIG. 6. This is because there is less energy in the peaks of the modulated burst. Finally, the indicated peak value is a weighted average of past peaks and hardly ever a true value for any actual peak due to the exponential nature of the capacitor charging required to replace the charge drained between peaks.
What is desired is a dual function peak metering circuit that satisfies both the monitoring and measuring functions for composite signals having both modulated and unmodulated components.